Discussion

We demonstrate in this video how to use the Fault Trajectory method to determine not only the dip and location of the fault at depth, but also the the variation in the shape of the fault with depth. As discussed earlier, the Fault Trajectory method in StructureSolver incorporates the differences between a structure’s hangingwall and footwall elevations when estimating the depth and dip of the fault.  Therefore, as we narrow the limits to include only the shallower part of the fault, we will get an estimate of the depth and dip of that part of the fault only.  This allows us to map the variation in the shape of the fault with depth. To show how the Fault Trajectory Method is sensitive to changes in fault shape, we will walk through area-depth analyses of two fault-related folds with varying fault geometries.   

The first example in this video uses the seismic section across a fault propagation fold from the Bermejo Basin shown in the previous video.  As noted in that video, the pre-growth horizons in the distal hangingwall are elevated above the footwall but nearly flat. So the structural relief in this part of the structure is positive but essentially constant. This observation is consistent with a planar but dipping fault at depth.

As we move the hangingwall regional limit across this portion of the structure, the analysis immediately shows that the computed fault follows the deep seismic reflector to within 200 m.

This approach can be applied in situations where observed folding implies a variable fault geometry, but the fault itself is not directly imaged. In the second example, we use a seismic section across the Inner Moray Firth (Virtual Seismic Atlas¹).  We show how to apply the Fault Trajectory method, together with the StructureSolver restoration and kinematic modeling tools, to estimate with confidence the dip and location of the fault.

In this example, the main fault is not well-imaged at depth and interpretation of the major growth fault in the section is limited by the lack of data in the lower section.  We start with the basic fault geometry and pre-growth horizon interpretation from the  Atlas.  We check the interpretation using the StructureSolver Restoration feature.  Restoration of  the upper pre-growth horizon reveals good seismic continuity in the restored section, which supports the interpreted correlations.

Visual analysis of the section shows gradual folding in the hangingwall that is maintained towards the right side of the image.  In contrast to the distal hangingwall of the Argentina structure, the continual change in structural relief suggests a smoothly varying fault shape at depth.  Structural relief between the hangingwall and footwall also decreases towards the right, indicating that the underlying fault may also flatten.

Area-depth analysis of the major growth fault estimates a shallower far-field dip than is indicated in the seismic. This is consistent with the hangingwall fold shape. 

Next, we map the fault location for progressively smaller hangingwall regional limits, as shown and determine the envelope for the fault location.  Using this envelope as a starting point, we create a kinematic forward model to test and refine the fault shape.  Within a short time, we have developed a model that reproduces the pre-growth fold geometry and is consistent with the results from the Fault Trajectory analysis.

¹ Virtual Seismic Atlas. www.seismicatlas.org. Survey: Fugro Inner Moray Firth IMF97 Profile #3